Various gas sensing devices are known which are relatively small and rugged and which are thus well suited for permanent installation in locations that are not easily accessible. Such devices include, for example, the series 10000-1 hydrocarbon sensors manufactured and sold by General Monitors, Inc., of Costa Mesa, Calif. These devices are often referred to as catalytic devices and can be typically mounted near or on the ceilings of rooms or mine shafts or inside ducts or flues, where access is difficult and in some instances dangerous. These devices provide signals that can be applied to suitable instruments located, for example, in a remote control room. The instruments display readings or indications of the gas concentrations detected by the gas sensing devices.
It is desirable to calibrate each such gas sensing device periodically to ensure that the data provided by the devices is accurate. In the past, the calibration has required that a worker carry a source of calibration gas such as a high-pressure tank or bottle to the sensing device. With a device such as one of th hydrocarbon sensing devices described above, the worker would first cover the device so that any hydrocarbon gas within the sensor would be consumed by the sensor, thereby allowing the sensor to be zeroed or calibrated with respect to an absence of hydrocarbon gas. Once this is done, the worker would then expose the sensor to calibration gas. The reading or indication provided by the sensor would then be calibrated with respect to the known concentration of the calibration gas.
As can be appreciated, this calibration process requires a worker to perform calibration operations at the sensing device itself. Where the device is not easily accessible, the calibration process becomes difficult and can expose the worker to dangerous environments. Moreover, the worker may be required to perform various manual tasks such as manipulating a tank of calibration gas at the sensing device while positioned on a ladder, further increasing the difficulty of the calibration task and also increasing the worker's exposure to hazardous working conditions. Because of the difficulty associated with performing the calibration process, gas sensing devices may not be calibrated as often as desirable, leading to inaccuracies and unreliable data from the gas monitoring system.
A remote calibrator device is described in application Ser. No. 314,363, entitled "Remote Calibrator, filed in the U.S. Patent and Trademark Office on Oct. 23, 1981 now abandoned (presently understood to be in an abandoned state). The calibrator device described in that application, which was assigned to the same assignee as the present application, was relatively expensive to manufacture and maintain, and subject to gas leakage over the wide temperature range within which the sensor and calibrator are expected to operate.
Thus, there is a need for a device which can be reliably used to remotely calibrate gas sensing devices with a minimum of worker time and without exposing the worker to dangerous environments or working conditions.